Hydraulic motors of the slow moving type are usually designed to be very sturdy and adapted to sustain hard stress. Therefore they have proved to be very suitable for the low speed turning of the rotary air preheater.
Therefore they have been mounted in the casing of the heat exchanger just outside the radial bearing with the rotating part of the hydraulic motor secured to the end of the hub. The hub is, however, subject to very high mechanical stress, partly caused by the strong temperature variations present in a regenerative heat exchanger. This shows itself a.o. by bending motions of the hub resulting in a tendency of the fastening of the hydraulic motor to the casing to break apart by radially directed forces emanating from the radial bearing.
This drawback has been eliminated by making the attachment of the motor flexible for radial forces and non-rotatable to all other forces, as shown in U.S. Pat. No. 3,687,192. This solution has proved not quite reliable due to endurance damages to the motor attachment.